1. Field of the Invention
The present invention is directed to a paper plate wherein the outwardly projecting rim of the container are formed by regions having four distinct radii of critically controlled configuration.
2. Description of Background Art
Fiberboard containers, such as paper plates and trays, are commonly produced either by molding fibers from a pulp slurry into a desired form of the container or by pressing a paperboard blank between forming dies into a desired shape. The molded pulp articles, after drying, are fairly strong and rigid but generally have rough surface characteristics and are not usually coated so that the containers are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers can be decorated and coated with a liquid-proof coating before being stamped by the forming dies into a desired shape. The products may be formed in many different shapes, for example, rectangular or polygonal as well as round and in multi-compartment configurations.
Pressed paperboard containers tend to have a somewhat reduced strength and rigidity as compared to containers made by a pulp molding process. Much of the strength and resistance to bending of a plate-like container made by either process lies in the sidewall and rim areas which surround the center or bottom portion of the container. In a plate-like structure made by a pulp molding process, the sidewall and overturned rim of the plate are unitary, cohesive structures which have good resistance to bending as long as they are not damaged or split. In contradistinction thereto, when a container is made by pressing a paperboard blank, a flat blank must be distorted and changed in area in order to form the blank into the desired three-dimensional shape. Score lines are sometimes placed around the periphery of the blanks being formed into deep pressed products to allow the paperboard to form or yield at the score lines to accommodate the reduction in area that takes place during pressing. However, the provision of score lines, flutes or corrugations in the blank may result in a formed product with natural fault lines about which the product will bend more readily, under less force, than if the product were unflawed. Shallow containers, such as paper plates, may also be formed from paperboard blanks which are not scored or fluted. However, the pressing operation will cause wrinkles or folds to form in the paperboard material at the rim and sidewalls of the container at more or less random positions. The folds act as natural lines of weakness within the container about which bending may occur.
In a common process for pressing paperboard containers from flat blanks, a sheet or web of paperboard is cut to form the blank, circular shape for a plate, and the blank is then pressed firmly between upper and lower dies which have die surfaces conforming to the desired shape of the finished container. The paperboard web stock is usually coated with a liquid-proof material on one surface and may also have decorative designs printed under the coating. The surfaces of the upper and lower dies have typically been machined such that when the dies begin to compress the shaped paperboard blanks between the dies, the die surfaces will be generally spaced uniformly apart over the entire surface area of the formed paperboard. The lower die may be spring mounted to limit the maximum force applied to the paperboard between the dies. If the spacing between the dies is uniform, the force is distributed over the entire area of the paperboard.